In strip handling apparatus, such as that preceding and following steel strip continuous annealing furnaces, closely controlled tension must be maintained on the strip entering and leaving the furnace because of diminution in tensile strength of the strip at annealing temperatures. Additionally, the hundreds of feet of steel strip in the continuous annealing furnace, depending on the type of steel, elongates at an average rate of about 1.2 inches per 100 feet of strip for each 180.degree. F. rise in temperature.
A conventional continuous annealing line is described and illustrated in the MAKING, SHAPING AND TREATING OF STEEL, published by United States Steel Corporation, 9th Edition at page 977. In this type of line the tension on the strip in the annealing furnace is controlled by having a free-hanging loop of strip ahead of the furnace and a free-hanging loop of strip following the furnace in relation to strip movement, with a drag tensioning roll or bridle roll unit at the strip exit side of the first loop for exerting controlled tension on the strip going to the furnace and a drive tension roll or bridle roll unit on the strip entry side of the second loop for pulling the strip through the furnace. The tension unit that pulls the strip through the furnace is driven by speed regulated or controlled electrical motors and the tension unit ahead of the furnace is connected to current controlled electrical generators which control the tension on the strip between the two tension units. The tension or bridle roll units incorporate a series of rolls around each of which the strip passes and each of these rolls acts to increase or decrease the tension on the strip approaching that roll. It will be apparent from the schematic diagram appearing in the aforementioned publication that the strip passing through the tension or bridle roll unit ahead of the furnace will have additional tension placed on the strip as the strip passes over each of the rolls in the tension unit with the tension placed on the strip by the final roll in the tension unit being the tension on the strip throughout the annealing furnace and up to the first roll in direction of strip travel of the tension roll unit following the furnace. This same phenomenon is reversed in the tension unit following the furnace.
In respect to each of these tension roll units, the strip entering the one ahead of the furnace from the free-hanging loop and the strip leaving the one following the furnace and entering the free-hanging loop can have appreciable tension on it by virtue of the weight of the strip in the respective free-hanging loop. The amount of strip in each loop keeps varying depending upon many factors but the important result in respect to the present invention is a resulting varying tension placed on the strip entering the tensioning roll unit ahead of the furnace and leaving the tensioning roll unit following the furnace. This varying tension adversely influences control of the tension on the strip being delivered from the former tension roll unit and on the strip entering the latter tensioning roll unit while at the same time introducing forces in the tensioning units which tend to cause strip slippage and strip weaving on the rolls of the tensioning units.
In the conventional strip handling line such as that illustrated in the aforementioned publication, the motors of the driving tension unit and the generators of the drag tension unit are in each case interconnected electrically so as to act in one case as a drive unit and in the other as a drag unit. As a result, under a given condition of strip gage, strip width and strip speed motor in the drive unit adds a controlled measure of tension to the strip backward relative to strip travel from the associated free hanging loop and each roll of the drag tension unit does the same forward relative to strip travel from the associated free hanging loop, whereby the varying weight of the strip in each free hanging loop is added to or subtracted from the desired tension to the detriment of accurate control of the tension and efficient strip tracking.